1. Field of the Invention
The present invention relates generally to a cam follower which includes a bearing unit mounted on a head portion of a cantilevered stud and each having rollers and an outer ring so that the outer peripheral surface of the outer ring constitutes a follower surface driven by a cam or cam-like guide rail (hereinafter xe2x80x9ccam-driven surfacexe2x80x9d). More particularly, the present invention relates to an improved cam follower suitable for use in indexing drive devices and the like.
2. Description of the Related Art
Among various known types of cam followers is one which, as illustrated in FIG. 5, includes a bearing unit mounted on a head portion of a cantilevered stud 51. The bearing unit includes rollers 52 and an outer ring 53 provided around the outer periphery of a distal end portion of the stud 51. In indexing drive devices, a plurality of such cam followers 50 are disposed at equal intervals or pitches around a driven shaft. Driving shaft is disposed orthogonally to the driven shaft, and the driving shaft has a helical cam formed on its outer periphery. The surface of the helical cam (hereinafter xe2x80x9ccam surfacexe2x80x9d) is capable of being sequentially brought into contact with one cam-driven surface to another. Specifically, as the driving shaft makes one rotation, the cam followers are caused to turn with the driven shaft by an angle equal to one pitch between the cam followers. In this way, the driven shaft is driven to turn by a given angle corresponding to the number of rotations of the driving shaft.
With the conventionally-known cam follower, the cam surface and cam-driven surface must of course be processed with high precision in order to attain predetermined linear contact with each other. However, if an inclination occurs in the cantilevered stud of the cam follower, the cam-driven would be brought in so-called xe2x80x9cuneven or biased contactxe2x80x9d with the cam surface, even in the case where the cam and cam-driven surfaces are processed with sufficient precision. In the case of an indexing drive device or the like, the main cause for such biased contact is that the cam followers project radially outward from the outer periphery of the driven shaft with the respective studs supported in a cantilever fashion and including the bearing units mounted on their head portions and thus the studs are apt to resiliently bend or incline due to the pressing force applied from the cam. Under such conditions, mechanical stress acting on the cam-driven surface and the rolling-contacting surfaces of the rollers and stud""s head portion in each of the cam followers would assume a distribution pattern such that it becomes greater in regions closer to the proximal end of the cantilevered stud.
The stud may be secured to the periphery of the driven shaft in a previously inclined posture at an initial stage of assemblage of the cam follower, taking into account the possible resilient bending or inclination; in this case too, however, if the stud is secured to the periphery of the driven shaft in more than or less than a predetermined initial inclination due to manufacturing and assembling errors, then the mechanical stress acting on the cam-driven surface and the rolling-contacting surfaces of the rollers and stud""s head portion would assume a distribution pattern such that it becomes greater in regions closer to the free (i.e., distal) or proximal end of the stud.
According to a xe2x80x9crated cam follower lifetimexe2x80x9d based on the Japanese Industrial Standards (JIS B 1518-1992), it is required that the mechanical stress acting on the rolling-contacting surfaces of the rollers and stud""s head portion be distributed uniformly. In cases where greatest mechanical stress occurs in a local region proximate to the free or proximal end of the cantilevered stud and decreases abruptly in a direction toward the other end, the actual lifetime of the cam follower would be shortened to a significant degree.
It is accordingly an object of the present invention to provided a cam follower which includes a cantilevered stud and a bearing unit having rollers and an outer ring mounted on a head portion of the cantilevered stud so that the outer peripheral surface of the outer ring constitutes a cam-driven surface, and which can effectively alleviate a maximum intensity level of mechanical stress adversely affecting the lifetime of the bearing unit.
It is another object of the present invention to provide a cam follower which includes a cantilevered stud and a bearing unit having rollers and an outer ring mounted on a head portion of the cantilevered stud so that the outer peripheral surface of the outer ring constitutes a cam-driven surface, and which can effectively level off mechanical stress adversely affecting the lifetime of the bearing unit.
According to a first aspect the present invention, there is provided a cam follower of the type comprising a cantilevered stud and a bearing unit mounted on a head portion of the stud and including rollers and an outer ring, an outer peripheral surface of the outer ring constituting a cam-driven surface. The cam-driven surface includes a central load bearing region substantially parallel to an axis of the cantilevered stud and a couple of load-reducing regions indented radially inward from the central load-bearing region and located on axial opposite sides of the central load-bearing region axially symmetrically to each other.
Because the cam-driven surface includes two radially-indented load-reducing regions formed axially symmetrically about the central load-bearing region, its effective length of contact with a cam surface is shortened as compared to the cam-driven surface of the conventional cam followers, and the pressing force from the cam or cam-like guide rail acts only on the central load-bearing region located at a given distance from the axial opposite ends of the cam-driven surface, instead of acting intensively on the axial opposite ends of the cam-driven surface. Further, in the present invention, the mechanical contact stress applied to the cam driven surface is not transmitted, as is, to between the inner surface of the outer ring and the outer surface of the roller and between the outer surface of the roller and the outer surface of the stud""s head portion; instead, the mechanical stress transmitted to between the inner surface of the outer ring and the outer surface of the roller and between the outer surface of the roller and the outer surface of the stud""s head portion are caused to present a maximum intensity level at points corresponding to the proximal end of one of the load-reducing regions and then gradually decreases in the directions toward the axial opposite ends of the roller. The load-reducing bearing regions may be formed by partly cutting away axial opposite end portions of the outer ring.
When the cantilevered stud is resiliently bent or inclined by the pressing force from the cam or when the cantilevered stud is inclined in any direction due to an assembling or manufacturing error, the mechanical stress acting on the cam-driven surface becomes greatest at a boundary between the central load-bearing region and one of the load-reducing regions and gradually decreases over the load-bearing region in a direction toward one of the axial opposite ends of the outer ring.
In the cam follower, distribution of the mechanical stress acting on the roller differs from distribution of the mechanical stress acting on the cam-driven surface. The inner peripheral surface of the outer ring and the outer peripheral surface of the stud""s head portion extend parallel to the axis of the stud, and each of the rollers has a central cylindrical portion parallel to the axis of the stud and a couple of crown portions extending axially from the axial opposite ends of the cylindrical portion while gradually decreasing in diameter in the directions toward the axial opposite ends of the roller. Although the outer ring and rollers are normally made of steel and have considerable rigidity, they can be resiliently deformed to a certain degree. Thus, the mechanical stress acting on the cam-driven surface is transmitted to between the inner or roller-rolling surface of the outer ring and the outer surface of the roller, from which it gradually decreases over a central range corresponding to the axial length of the central cylindrical portion of the roller and then over end ranges corresponding to the axial length of the crown portions of the roller. The mechanical stress acting on the cam-driven surface is also transmitted to between the outer or roller-rolling surface of the stud""s head portion and the outer surface of the roller and acts in generally the same manner as the above-mentioned mechanical stress transmitted to between the inner or roller-rolling surface of the outer ring and the outer surface of the roller.
By thus providing the radially-indented load-reducing bearing regions on opposite sides of the central load-bearing region of the cam-driven surface, the maximum mechanical contact stress applied to the cantilevered stud is initially transmitted only to the central portion of the roller and then gradually decreases in the directions toward the axial opposite ends of the roller, so that the mechanical stress acting between the inner surface of the outer ring and the outer surface of the roller and the mechanical stress acting between the outer surface of the roller and the outer surface of the stud""s head portion can be effectively leveled off. Because the maximum mechanical stress acting between the inner surface of the outer ring and the outer surface of the roller and the maximum mechanical stress acting between the outer surface of the roller and the outer surface of the stud""s head portion are transmitted only to the central portion of the roller, the maximum intensity level of the mechanical stress on the outer ring, roller and stud can be significantly alleviated as compared to the conventional cam followers where the maximum mechanical stress occurs at the axial opposite ends of the roller.
Generally speaking, the greatest factor that governs the lifetime of the bearing unit is scaling-off of the wall surface due to fatigue of the metal material. In such a situation where the stud is cantilevered and thus the mechanical stress tends to concentrate at one end of the outer ring, the actual lifetime of the bearing unit can be prolonged by the present invention where the two load-reducing regions are formed on both sides of the central load-bearing region so as to level off the mechanical stress acting on the roller and alleviate the maximum level of the mechanical stress.
In each of the conventional bearing units, the outer ring normally has corner edges rounded or chamfered as typically defined by JIS (Japanese Industrial Standards) representation xe2x80x9cRxe2x80x9d or xe2x80x9cCxe2x80x9d. In the inventive cam followers, the load-reducing regions of the outer ring has greater chamfers than those conventional chamfers.
The inventive cam follower is also characterized in that the load-reducing regions are formed on both sides of the central load-bearing region axially symmetrically to each other about the load-bearing region. This symmetric arrangement can level off the mechanical stress on the bearing unit and also effectively alleviate the maximum intensity level of the stress, irrespective of a direction in which the cantilevered stud is resiliently bent or inclined relative to the cam or cam-like guide rail.
Further, the symmetrical provision of the load-reducing bearing regions can eliminate a need to accurately place the outer ring in a predetermined one of two axial orientations at the time of assemblage of the cam follower; that is, the outer ring can be mounted in either of the two axial orientations. Namely, if the stud is always resiliently bent or inclined only in one known direction, then the cam follower may have the load-reducing region only on one side of the central load-bearing region, in which case, however, there would arise a need to first accurately orient the outer ring so that the load-reducing bearing region lies adjacent to the free or proximal end of the cantilevered stud before the outer ring is mounted on the stud. As a result, the symmetrical provision of the load-reducing bearing regions can reliably avoid erroneous mounting of the outer ring and also simplify the necessary assembling operations.
According to a second aspect of the present invention, the cam-driven surface includes a central load-bearing region substantially parallel to an axis of the stud, and a couple of load-reducing regions located on axial opposite sides of the central load-bearing region axially symmetrically about the central load-bearing region and having respective outer diameters decreasing in the directions toward the axial opposite ends of the outer ring.
Because the load-reducing regions are provided on axial opposite sides of the central load-bearing region axially symmetrically to each other about the central load-bearing region and have their respective outer diameters decreasing in the directions toward the axial opposite ends of the outer ring, the mechanical contact stress applied from the cam or cam-like guide rail to the outer ring presents a maximum intensity level at the proximal or free end of one of the load-reducing regions and is gradually reduced over the one load-reducing region. The load-bearing region has a cylindrical surface. The load-reducing regions are preferably made by shaping the outer ring""s axial opposite end portions in such a manner that the end portions each assume a section with a secondary degree curve, an n-degree curve, an exponential function line, a trigonometric function line, or the like. In any case, the non-load-bearing regions have a reduced mean diameter smaller than the outside diameter of the load-bearing regions. The specific shape of the load-reducing regions is determined taking into account possible manufacturing errors of the cam or cam-like guide rail and various components of the cam follower, mechanical strength of the stud, etc.
When the cantilevered stud is resiliently bent or inclined due to the pressing force from the cam or when the cantilevered stud is inclined in any direction due to an assembling or manufacturing error, the mechanical stress acting on the cam-driven surface in the inventive cam follower becomes greatest at a boundary between the central load-bearing region and one of the load-reducing regions and gradually decreases in the directions toward the axial opposite ends of the outer ring, if the load-reducing regions are each shaped to have a conical surface.
If the load-reducing regions are each formed to have a curved surface and when the cantilevered stud is resiliently bent or inclined, the point of the maximum mechanical stress acting on the outer ring is displaced toward one of the axial opposite ends of the outer ring by the resilient bending or inclination; in this case, the maximum mechanical stress acts on a point close to the boundary between the central load-bearing region and one of the load-reducing regions and gradually decreases in the directions toward the axial opposite ends of the outer ring. It is most preferable that the load-reducing regions be formed, taking into consideration of possible bending or inclination of the stud, in such a way that the mechanical stress on the outer ring can be leveled off appropriately; however, it is normally very difficult to predict manufacturing and assembling errors of various components parts of the cam follower. Therefore, it is desirable that the load-reducing regions be formed to have a curved surface to allow the maximum mechanical stress to occur at a point near the boundary.
In the cam follower according to the second aspect of the invention too, distribution of the mechanical stress acting on the roller differs from distribution of the mechanical stress acting on the cam-driven surface. As in the above-discussed first-aspect cam follower, the mechanical stress acting between the inner or roller-rolling surface of the outer ring and the outer surface of the roller and the mechanical stress acting between the outer surface of the roller and the outer or roller-rolling surface of the stud""s head portion are caused, via the load-reducing regions, to gradually decrease or level off.
Further, because the maximum mechanical stress acting between the inner surface of the outer ring and the outer surface of the roller and the maximum mechanical stress acting between the outer surface of the roller and the outer surface of the stud""s head portion are transmitted only to the axial central portion of the roller, the maximum intensity level of the mechanical stress on the outer ring, roller and stud can be significantly alleviated as compared to the conventional cam followers where the maximum mechanical stress occurs at the axial opposite ends of the roller.
The actual lifetime of the bearing unit can be prolonged by the present invention where the load-reducing regions are provided at the axial end portions of the cam-driven surface to level off the mechanical stress acting on the roller and alleviate the maximum intensity level of the mechanical stress.
In the second-aspect cam follower too, each of the load-reducing regions of the outer ring has greater chamfers than those in the conventional cam followers.
Furthermore, because the load-reducing regions are formed on both sides of the central load-bearing region axially symmetrically to each other about the load-bearing region, it is possible to level off the mechanical stress on the bearing unit and effectively alleviate the maximum intensity level of the stress, irrespective of a direction in which the cantilevered stud is resiliently bent or inclined relative to the cam or cam-like guide rail. Further, the symmetrical provision of the load-reducing regions can reliably avoid erroneous mounting of the outer ring without having to accurately place the outer ring in the predetermined axial orientation, and also simplify the necessary assembling operations.
The above and other objects, features and advantages of the present invention will become manifest to those versed in the art upon making reference to the following detailed description and the accompanying sheets of drawings in which certain preferred structural embodiments incorporating the principle of the present invention are shown by way of illustrative examples.